This invention relates to accelerometers, and is more particularly, but not exclusively, concerned with accelerometers for use in down-hole instrumentation for surveying a borehole.
U.S. Pat. No. 4,047,439 discloses an accelerometer comprising a housing defining a chamber, a magnetizable fluid within the chamber, a permanent magnet magnetically suspended within the chamber by the magnetizable fluid with its poles oriented along a displacement axis and displaceable from a null position along the displacement axis by an applied force, and sensing means for detecting displacement of the permanent magnet along the displacement axis and for supplying an electric signal indicative of the applied force.
Such an accelerometer requires to be calibrated prior to use. However, it is found that the required calibration of the accelerometer can tend to drift under the conditions of high temperature and vibration encountered down-hole, and this can lead to inaccuracy in measurement. It is believed that such drift is caused by changes in the effective mass of the proof mass of the accelerometer due to changes in the distribution of the magnetic particles within the fluid and in the magnetic interaction between these particles and the magnet.
It is an object of the invention to provide a novel form of accelerometer which is capable of improved performance under such conditions.